1. Field of the Invention
The invention relates to a ferroelectric integrated circuit having low sensitivity to hydrogen exposure and to a method for fabricating such a circuit.
2. Statement of the Problem
Ferroelectric compounds possess favorable characteristics for use in nonvolatile integrated circuit memories. See Miller, U.S. Pat. No. 5,046,043. A ferroelectric device, such as a capacitor, is useful as a nonvolatile memory when it possess desired electronic characteristics, such as high residual polarization, good coercive field, high fatigue resistance, and low leakage current. Lead-containing ABO.sub.3 -type ferroelectric oxides such as PZT (lead titanate zirconate) and PLZT (lanthanum lead titanate zirconate) have been studied for practical use in integrated circuits. Layered superlattice material oxides have also been studied for use in integrated circuits. See Watanabe, U.S. Pat. No. 5,434,102. Layered superlattice material compounds exhibit characteristics in ferroelectric memories that are orders of magnitude superior to those of PZT and PLZT compounds. Integrated circuit devices containing ferroelectric elements are currently being manufactured. Nevertheless, the persistent problem of hydrogen degradation during the manufacturing process hinders the economical production in commercial quantities of ferroelectric memories and other IC devices using the layered superlattice material compounds with the desired electronic characteristics.
A typical ferroelectric memory device in an integrated circuit contains a semiconductor substrate and a metal-oxide semiconductor field-effect transistor (MOSFET) in electrical contact with a ferroelectric device, usually a ferroelectric capacitor. A ferroelectric capacitor typically contains a thin film containing ferroelectric metal oxide located between a first, bottom electrode and a second, top electrode, the electrodes typically containing platinum. During manufacture of the circuit, the MOSFET is subjected to conditions causing defects in the silicon substrate. For example, the CMOS/MOSFET manufacturing process usually includes high energy steps, such as ion-mill etching and plasma etching. Defects also arise during heat treatment for crystallization of the ferroelectric thin film at relatively high temperatures, often in the range 500.degree.-900.degree. C. As a result, numerous defects are generated in the single crystal structure of the semiconductor silicon substrate, leading to deterioration in the electronic characteristics of the MOSFET.
To restore the silicon properties of the MOSFET/CMOS, the manufacturing process typically includes a hydrogen annealing step, in which defects such as dangling bonds are eliminated by utilizing the reducing property of hydrogen. Various techniques have been developed to effect the hydrogen annealing, such as a forming-gas anneal ("FGA"). Conventionally, FGA treatments are conducted under ambient conditions in a H.sub.2 --N.sub.2 gas mixture between 350.degree. and 550.degree. C., typically around 400-450.degree. C., for a time period of about 30 minutes. In addition, the CMOS/MOSFET manufacturing process requires other fabrication steps that expose the integrated circuit to hydrogen, often at elevated temperatures, such as hydrogen-rich plasma CVD processes for depositing metals and dielectrics, growth of silicon dioxide from silane or TEOS sources, and etching processes using hydrogen and hydrogen plasma. During processes that involve hydrogen, the hydrogen diffuses principally through the top electrode to the ferroelectric thin film, but also from the side edges of the capacitor, and reduces the oxides contained in the ferroelectric material. The absorbed hydrogen also metallizes the surface of the ferroelectric thin film by reducing metal oxides. As a result of these effects, the electronic properties of the capacitor are degraded. After the forming-gas anneal (FGA), the remnant polarization of the ferroelectrics is very low and no longer suitable for storing information. An increase in leakage currents also results. In addition, the adhesivity of the ferroelectric thin film to the upper electrode is lowered by the chemical change taking place at the interface. Alternatively, the upper electrode is pushed up by the oxygen gas, water, and other products of the oxidation-reduction reactions taking place. Thus, peeling is likely to take place at the interface between the top electrode and the ferroelectric thin film. In addition, hydrogen also can reach the lower electrode, leading to internal stresses that cause the capacitor to peel off its substrate. These problems are acute in ferroelectric memories containing layered superlattice material compounds because these oxide compounds are particularly complex and prone to degradation by hydrogen-reduction.
A related problem encountered in the fabrication of ferroelectric devices is the stress arising in and between the different circuit layers as a result of the manufacturing processes. The ferroelectric compounds comprise metal oxides. The products of the reduction reactions cause an increase in the total volume of the ferroelectric element. As a result, the ferroelectric thin film exerts an upward pressure on the layers above it.
Several methods have been reported in the art to inhibit or reverse hydrogen degradation of desired electronic properties in ferroelectric oxide materials. Oxygen-recovery annealing at high temperature (800.degree. C.) for about one hour results in virtually complete recovery of the ferroelectric properties degraded by hydrogen treatments. But the high-temperature oxygen-anneal itself might generate defects in silicon crystalline structure, thereby offsetting somewhat the positive effects of any prior forming-gas anneal on the CMOS characteristics. Also, a high-temperature oxygen-anneal may only be conducted prior to aluminum metallization. Furthermore, if hydrogen treatments have caused structural damage to the ferroelectric device, such as peeling, then a recovery anneal is not able to reverse effectively the damage.
To reduce the detrimental effects of the hydrogen heat treatment and protect the ferroelectric metal oxide element, the prior art also teaches the application of hydrogen barrier layers to inhibit the diffusion of hydrogen into the ferroelectric or dielectric material. The barrier layer is typically applied over the ferroelectric element, but it can also be applied below and to the sides of the element. Typically, hydrogen barrier layers are not completely effective in preventing hydrogen diffusion. Thus, even when a hydrogen diffusion barrier is used, it is not uncommon for structural damage to arise in the ferroelectric device and for hydrogen to reach the ferroelectric layer and degrade the ferroelectric properties of the ferroelectric material.
Therefore, it would be useful to have an integrated circuit with a ferroelectric memory device and a method for making such a circuit that would enhance the benefits of various measures used to protect ferroelectric oxide material, in particular, ferroelectric layered superlattice materials, from hydrogen degradation, while minimizing the complexity of the integrated circuit and its fabrication method.
3. Solution to the Problem
The invention solves the above problems by providing an integrated circuit in which at least one protective layer of a ferroelectric integrated circuit contains a small amount of oxygen. A protective layer can be a hydrogen barrier layer, a metallized wiring layer, or another layer or structural component of a ferroelectric integrated circuit. A ferroelectric integrated circuit according to the invention typically comprises a distinct hydrogen barrier layer containing a small amount of excess oxygen, or a metallized wiring layer containing a small amount of excess oxygen, or both. The invention also provides a method for forming a hydrogen barrier layer and a metallized wiring layer containing small amounts of oxygen. The small amount of oxygen in a protective layer according to the invention forms oxides to protect ferroelectric oxide material from hydrogen degradation. The oxides present in the protective layer protect ferroelectric oxides by reacting with hydrogen so that it does not diffuse into the ferroelectric material. In addition, the formation of the oxides in a hydrogen barrier layer directly over a ferroelectric circuit element exerts a compressive stress in a downward direction. The reduction of oxides by hydrogen in the ferroelectric material and other hydrogen reactions in the ferroelectric element cause stresses in the upward direction and can result in peeling of the thin films from the substrate. The compressive stress exerted by the hydrogen barrier layer in the downward direction as a result of oxide formation balances the upward stress exerted by the underlying ferroelectric thin film. The oxides in the hydrogen barrier layer thereby bring the capacitor stack into a stress equilibrium condition below catastrophic failure.
One aspect of the invention is the presence of small amounts of oxides near the surfaces of the protective layer. The oxides serve as "getters" of hydrogen. The small amount of oxides in the regions of the protective layer near its surfaces does not significantly decrease the electrical conductivity of the layer.
Preferably, the protective layer comprises a hydrogen barrier layer directly over a ferroelectric element. Preferably, the hydrogen barrier layer comprises a nitride of titanium or silicon if the hydrogen barrier layer is electrically conductive.
Another aspect of the invention is the presence of small amounts of oxides in metallized wiring layers directly over the ferroelectric element. The oxides are located in the regions near the surfaces of the metallized wiring layers.
Preferably, the metallized wiring layer comprises aluminum.
Another aspect of the invention is a protective layer containing a small amount of oxygen that covers the lateral side of a ferroelectric element.
A further aspect of the invention is the formation of a protective layer in such a manner that there is an oxygen gradient in the layers, with no oxygen present in the interior of the layers. Preferably, the concentration of oxygen is about two weight percent near the surfaces of the protective layer. Typically, the protective layer comprises a hydrogen barrier layer or a metallized wiring layer. If the protective layer comprises polycrystalline material with a plurality of crystal grains, such as a metal or ceramic material, then the small amount of oxygen is just enough to decorate the grain boundaries.
A further aspect of the invention is exertion of a downward stress by a hydrogen barrier layer containing a small amount of oxygen, caused by the oxides formed therein. The downward stress balances the upward stress exerted by the layers in the ferroelectric element as a result of hydrogen reactions.
A further aspect of the invention is a fabrication method in which small, varying amounts of oxygen are added to the sputtering gas atmosphere during sputter deposition of the protective layers so that oxygen is included in the regions near the surfaces of the layers, but not in the center region of the layers.
Numerous other features, objects and advantages of the invention will become apparent from the following description when read in conjunction with the accompanying drawings.